The present invention relates to violet dye mixtures for shading paper.
Optically brightened white paper is customarily shaded in commercial practice using violet dyes of high brilliance whose hue lies within a closely defined domain. Some of these dyes do afford the shade sought, but have certain undesirable properties. For instance, the water solubility of their sodium salts is poor, necessitating complicated measures to convert them into more soluble salts if concentrated aqueous formulations are to be produced, and/or their dilute stock solutions tend to throw substantial precipitates even in water of just 10xc2x0 German hardness. It is true that modifying the dyes by introducing an additional sulfo group reduces the disadvantages mentioned, but it also leads to an unwelcome reduction in affinity.
There is therefore a need for violet dyes for shading paper which afford the shade sought without having the aforementioned disadvantages. Shading is to be understood as meaning the treating with dyes in such an amount that, although the dyeing obtained is virtually invisible, other properties, for example the brilliance or brightness of white paper, are improved. The amounts of dye used are approximately between 0.5 and 100 g, preferably between 1 and 50 g, of pure dye per tonne of paper.
It has now been found that certain dye mixtures have the desired properties. Surprisingly, at the concentrations used in shading, the brilliance of the dyeings obtained is similar to that obtained with the individual components, whereas it is known that dye mixtures normally produce distinctly duller dyeings. The dye mixtures of the invention are additionally notable for good water solubility and stability to hard water.
The present invention accordingly provides violet dye mixtures for shading paper, comprising a violet dye of the formula 
where
R1 is hydrogen or alkyl,
R2 is hydrogen, alkyl or substituted or unsubstituted aryl,
R3 and R4 are independently hydrogen or alkyl,
X and Y are independently hydrogen, alkyl or alkoxy,
n is 1 or 2, and
Me is hydrogen or one equivalent of a colourless cation,
and sufficient of a red, blue or further violet anionic azo dye that this dye mixture produces in the Cielab colour co-ordinates system a relative hue angle of 270 to 295xc2x0 when the dyeing contains 0.00005 to 0.005% by weight of dye, based on the weight of the paper, with the proviso that the value of 0.005% is excluded.
Furthermore, suitable weight ratios of the violet dye of formula (1) or (2) to the red, blue or further violet anionic azo dye, according to the invention, have been found to lie within the range of between 98 to 2 parts and 40 to 60 parts, ratios of between 95 to 5 parts and 60 to 40 parts being preferred.
The dye mixture according to the invention is preferably employed in an amount to produce a dyeing which contains between 0.00005 and 0.004%, more preferably between 0.00005 and 0.003% and, most preferably, between 0.001 and 0.002% by weight of dye, based on the weight of the paper.
In the dyes of the formulae (1) and (2), alkyl is especially C1-C4alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl and tert-butyl.
Alkoxy is in particular C1-C4alkoxy, for example methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, sec-butoxy, isobutoxy and tert-butoxy.
Aryl is for example naphthyl or especially phenyl, and these radicals may be substituted, for example by alkyl, alkoxy, sulfo, carboxyl, halogen, alkylcarbonylamino or alkoxycarbonyl.
Me is hydrogen or one equivalent of a colourless cation, for example lithium, sodium, potassium, ammonium or the protonated form of a C4-C12trialkylamine, of a C4-C12diamine, of a C2-C15alkanolamine or of a polyglycolamine. Preferably Me is hydrogen or sodium.
The absolute hue angle is defined in the Cielab system by the colour co-ordinates a* and b* (Colorimetry, second Edition, Publication CIE No. 15.2 (1986). The values in the present application relate to the relative hue angle, which is based on the colour co-ordinates of the unshaded and unbrightened base paper. When calculating the relative hue angle, the absolute a* and b* values are replaced by the differences from the base paper (a*xe2x88x92a*0) and (b*xe2x88x92b*0). All the values of the relative hue angle are based on the small dye quantities which are used for shading white paper.
The process of the invention is preferably carried out using a dye mixture of such composition that a paper dyeing with this dye mixture in the stated concentration produces a relative hue angle of 275 to 285xc2x0.
Particularly preferred violet dyes conform to the formula 
wherein
X is hydrogen or C1-C2alkyl,
Y is hydrogen or C1-C2alkoxy,
R1 is hydrogen or C1-C2alkyl,
R2 is hydrogen, C1-C2alkyl or phenyl,
R3 and R4 are independently hydrogen or C1-C2alkyl, and
Me is hydrogen, sodium, lithium or an organic ammonium or alkanolammonium cation.
Of these dyes, particular preference is given to the violet dyes of the formula 
and of the formula 
wherein
X and Y are independently hydrogen, C1-C2alkyl or C1-C2alkoxy,
R1 is hydrogen or C1-C2alkyl,
R2 is hydrogen, C1-C2alkyl or phenyl, and
Me is hydrogen, sodium or an organic ammonium or alkanolammonium cation.
The second component included in the dye mixtures of the invention is a red, blue or further violet dye which shifts the shade of the dyeing with the dye of the formula (1) or (2) in such a way that the net effect is to produce a desired relative hue angle of 270 to 295xc2x0. Suitable dyes are in particular the anionic dyes used in the paper industry. In general, such dyes are classified as Direct Dyes, as exemplified in the Colour Index and are anionic mono- or, especially, bis-azo compounds.
Preference is given to red dyes of the formula 
or of the formula 
whilst especially useful further violet anionic azo dyes are those of the formula 
and blue anionic azo dyes those of the formula 
wherein
Me is hydrogen, sodium or an organic ammonium or alkanolammonium cation.
The violet dyes of the formulae (1) to (6) and (9) and also the red dyes of the formulae (7) and (8) and the blue shading dyes of formula (10) are known or can be prepared in a manner known per se.
Especially useful combinations of dyes according to the invention are, for example,
C.I. Direct Violet 35 together with C.I. Direct Red 254, C.I. Direct Red 227,
C.I. Direct Violet 26 or C.I. Direct Violet 51,
C.I. Direct Violet 26 together with C.I. Direct Blue 67, C.I. Direct Blue 279 or
C.I. Direct Blue 290 and
C.I. Direct Violet 51 together with C.I. Direct Blue 67, C.I. Direct Blue 273 or
C.I. Direct Blue 290.
The dye mixtures of the invention are particularly useful for shading paper, especially paper containing optical brightener. They are preferably used as solid or liquid commercial form.
The pulverulent or granular form of the dye mixtures is used particularly in batchwise pulp dyeing where the dye mixture, customarily in the form of a stock solution, is added in the pulper, in the beater or in the mixing chest. Preference is here given to using dye preparations which as well as the dye mixtures, may further include extenders, for example urea as solubilizer, dextrin, Glauber salt, sodium chloride and also dispersants, dust-proofing agents and sequestrants, such as tetra-sodium phosphate.
The present invention accordingly further provides solid dye preparations for dyeing paper which include a dye mixture according to the invention.
In recent years, the use of concentrated aqueous solutions of dyes has gained importance because of the advantages possessed by such solutions when compared with dyes in powder form. The use of solutions avoids the difficulties associated with dust formation and releases the user from the time-consuming and frequently difficult dissolving of the dye powder in water. The use of concentrated solutions was also prompted by the development of continuous dyeing processes for paper, since it is convenient in these processes to meter the solution directly into the pulp stream or to add it at some other suitable point of the paper-making process.
The present invention accordingly further provides concentrated aqueous dye mixture solutions for dyeing paper which contain from 5 to 30% by weight, based on the total weight of the solution, of a dye mixture according to the invention.
The concentrated solutions preferably contain a low level of inorganic salts, which may be achieved, if necessary, by known methods, for example reverse osmosis.
The concentrated solutions may include further auxiliaries, for example solubilizers such as xcex5-caprolactam or urea, organic solvents, for example glycols, polyethylene glycols, dimethyl sulfoxide, N-methylpyrrolidone, acetamide, alkanolamines or polyglycolamines.
In the Examples hereinbelow, parts and percentages are by weight and temperatures are recorded in degrees Celsius. The reported quantities are based on pure dye, reckoned as free acid.